The present invention relates to stereoscopic test instrumentation, and more particularly to a stereoscopic extinction ratio test pattern for Left/Right (L/R) crosstalk evaluation.
One of the problems with stereoscopic three-dimensional (3D) video and 3D movies is crosstalk between the left and right eye images. 3D videos and movies require two separate images, one for viewing by the left eye and the other for viewing with the right eye. When the two images are shown together, the viewer sees a stereoscopic 3D image. When one eye is occluded in some manner from seeing the 3D image, what should be seen is only the image intended for the viewing eye. However there is always some L/R crosstalk where some of the left eye image is seen by the right eye when the left eye is occluded and vice versa, giving an appearance of “ghosting.” The ghosting occurs when the occlusion technique, which is used for assuring that each eye only sees the image intended for that eye, does not sufficiently attenuate the image for the other eye. The technologies for stereoscopic 3D provide different degrees of crosstalk, but none are completely free from this problem. The current technologies are discussed below.
An “active shutter” method time-multiplexes the left and right images on a display, such as liquid crystal diode (LCD), cathode ray tube (CRT), projector/screen displays and the like, and active shutter glasses are worn by the viewer. The image rate is above 60-120 Hz, and well above the flicker fusion frequency of the human eye, i.e., no large area flicker. The active shutter glasses are synchronized with the display so when the shutter is active for the left eye only the left eye image is shown on the display and vice versa. In other words, only the proper image is seen by each eye—the left eye sees only the left image and the right eye sees only the right image. This technology provides the best isolation and minimum ghosting or L/R crosstalk, since the active shutter glasses block the light very well, although not completely. Many new television sets already have this capability. However the active shutter glasses are bulky, and present a problem for those viewers who already wear prescription glasses, as opposed to contact lenses. This may be acceptable for short periods in a movie theater, but may not be acceptable when viewing television or movies in the privacy of the viewers' own living room.
Another technique is a polarization technique using passive glasses. In this method the video or displayed light image is a composite of two orthogonal polarized images. The left and right eye images are displayed at the same time with clockwise and anti-clockwise, circular polarization respectively in the more recent methods. +45 and −45 degree linear polarization also has been used in the past, the common factor being that the polarization is orthogonal. However the circular polarization technique performs better on some projection screens and the crosstalk is not dependent upon the head tilt of the viewer. Currently micro-polarized LCD laptop and desk-top panel displays are available where each line alternates in the twist of circular polarization. Also movie theaters superimpose the left and right images with opposing twist of circular polarization using special silver screens that retain the polarization. In both cases passive glasses, that are much less bulky than the active shutter glasses, are used, where each lens is a circular polarizer of opposite twist. Since these polarized lenses are not perfect, and the special silver screen in theaters is not perfect, some of the left image leaks into the right eye and vice versa, creating ghosting or crosstalk.
A third anaglyphic technique uses color-filter passive glasses. In this method the light images from the display contain left and right images that are also superimposed, but separately colored, so that one image is a complement color to the other. The anaglyph glasses have opposing color filters for each eye lens that pass one color and block the other. Some are either magenta/green, as in the digital video disc (DVD) of the movie “Coraline”, for example, or more commonly red/cyan. The anaglyph glasses are very cheap and current two-dimensional (2D) color displays, such as television sets, projectors, etc., may be used. This provides a distinct advantage over the polarization and active shutter methods described above. However the anaglyph glasses are not good for color video/film and the filters on each eye in the glasses have even more crosstalk. This makes the ghosting even worse.
Another technique or technology for displaying stereoscopic 3D does not require any glasses at all, and is commonly called “auto-stereoscopic.” Several methods exist, with the most common based on a parallax barrier, pixel overlay that blocks the left eye view from the image pixels for the right eye view, and vice versa. See http://en.wikipedia.org/wiki/Autostereoscopy#Parallax barrier. These displays do not require the viewer to wear glasses, but most have a limited viewing zone in both angular position as well as viewer distance from the display. A primary distortion that occurs as the viewer moves away from the center of the optimal viewing zone is crosstalk, as discussed above. In fact, the limits of usable display zones are sometimes specified in terms of crosstalk, such as less than five percent (5%) crosstalk within an inner zone and less than fifteen percent (15%) crosstalk over a wider viewing zone.
The crosstalk or image-extinction ratio between the left and right eyes in both the polarized and anaglyphic methods depends on several factors—the quality of the glasses (polarizer or band-pass color filters) and, in projection displays, the ability of the screen to retain the polarization or maintain the correct colorimetry. For theater displays polarization crosstalk may also vary with viewing angle or screen uniformity/quality. As discussed at a Society of Motion Picture and Television Engineers (SMPTE) conference, L/R crosstalk of less than one percent (1%) is desired, but seldom achieved, for 3D movie theaters.
What is desired is a simple test pattern for evaluating L/R crosstalk in a stereoscopic 3D display that is useful for all current techniques discussed above.